The present invention relates to an improvement in the processing of oilseeds such as, for example, soybean, cottonseed, corn, peanut, safflower, sunflower, and palm. More particularly, the invention relates to an improved process for the extraction of oils from oilseeds utilizing an isopropyl alcohol-based extraction solvent.
The oilseed industry of the United States produces on an annual basis about thirteen million tons of seed oils from roughly one billion bushels of seed crops, predominantly soybean and cottonseed. Essentially all of this oil is recovered from the seeds by solvent extraction. The oils find primary use in foods, e.g., shortening, margarine, cooking oils, and salad oils, while seed meal from which the oil has been extracted, having a high protein content, is generally processed into animal feeds. About two percent of this meal is further refined for human consumption.
In the recovery of seed oils, the industry consumes large quantities of extraction solvent. Hexane has long been recognized as the standard solvent in the industry, due in part to its low cost relative to other solvents and in part to its physical and chemical properties. However, in recent years incentive has developed for the replacement of hexane as the solvent of choice for oilseed extraction. Increasing hexane costs and possibilities of supply shortages account for some of this incentive. In addition, for health and safety reasons, solvent specifications in the industry, precautions to prevent exposure of workers to hexane, and relevant hydrocarbon emission standards may be tightened. Furthermore, interest in producing an upgraded seed meal, for instance a meal suitable for human consumption without the need for secondary extraction or other refining, has given rise to attempts at use of alternate extraction solvents.
Of particular relevance to the present invention is the prior art relating to extraction of oilseeds with an isopropanol-based solvent. Characteristics of isopropanol extraction are generally well known and are described, for instance, by Harris et al in a three-part publication entitled "Isopropanol as a Solvent for Extraction of Cottonseed Oil" (J. Am. Oil Chem. Soc., November 1947, Vol. 24, p. 370-375; December 1949, Vol. 26, p. 719-723; and July 1950, Vol. 27, p. 273-275). More recent developments in oilseed processing with isopropanol are outlined by Youn and Wilpers in U.S. Pat. No. 4,298,540.
It is recognized that, although isopropanol is in many respects attractive for use as an oilseed extractant, it cannot as a practical matter be directly substituted in a process designed for use of a hexane solvent. Direct substitution would, for example, entail substantial increases in process energy requirements. Under conventional hexane extraction processing, solvent is recovered from both the oil and meal products by evaporation. However, because energy necessary for this evaporation is a major factor in processing costs, because the heat of vaporization of an isopropanol-water azeotrope is nearly three times that of hexane, and because isopropanol extraction typically requires greater quantities of solvent than hexane extraction, it is not economically feasible to practice a like evaporation of isopropanol-based solvent. The above-referenced publications of Harris et al describe a method for phase separating solvent and extracted oil in the extraction process miscella which has advantages in energy conservation over evaporative separation. However, these publications do not address aspects of the problem relating to the energy intensive separation of solvent from extracted meal.
It is an object of this invention to enhance the energy efficiency of solvent recovery in an oilseed extraction process utilizing an isopropanol-based solvent. It is a further object of the invention to improve the efficiency of the extraction of oils from oilseeds.
In most general terms, for realization of these objects the present invention relies in part upon a mechanically-aided drainage of solvent from oilseed meal following extraction of the seeds with an isopropanol-based solvent. The prior art is not known to have considered such drainage for liquid removal from meals extracted with isopropanol-based extraction solvents. For conventional extraction processes utilizing a hexane solvent, it has been observed (H. P. J. Jongeneelen, J. Am. Oil Chemists' Soc., June 1976 (Vol. 53), pp. 291-292) that liquid hold-up in the meal can be reduced by gravity drainage or pressing. However, it is at the same time observed that drainage of liquid from the meal in hexane processing has a drawback from the standpoint energy utilization and process integration. It is suggested that mechanical drainage of solvent, as opposed to gravity drainage, which is generally conducted within the conventional extraction, is of no advantage to the extraction process unless practiced in conjunction with special provisions for the distillative separation of solvent from extracted oil. Such considerations of distillative separation do not apply in isopropanol solvent extraction processes in which solvent is separated from oil primarily by phase separation rather than by distillation. In other aspects, the invention relates to criticalities in the degree of solvent drainage from the meal and to particular routings of process flows.